Bump-on-Trace (BOT) structures were used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is low. The conventional BOT structures may achieve the same reliability as the conventional bond structures based on metal pads.
The BOT structures used to include a solder mask layer that is formed on the metal traces. The solder mask layer covers portions of the metal traces, and leaves some openings, through which the metal traces are exposed. During the bonding process, solder bumps extend into the openings, and are bonded to the exposed portions of the metal traces. The solder mask layer provides mechanical support for the BOT structures, and the metal traces are unlikely to peel off from the underlying structures.
With the evolving of bump structures, the solder mask layer may be omitted. Since the package substrates may be bonded to device dies that include extreme low-k dielectric layers therein, not forming the solder mask layer is beneficial for reducing the risk of the peeling occurring in the extreme low-k dielectric layers. However, without a solder mask layer to cover the metal traces, the risk of the peeling between the metal traces and the underlying structures in the package substrates increases.